1. Field of the Invention
This invention relates generally to semiconductor processing, and more particularly to methods and apparatus for reducing crosstalk in circuit boards.
2. Description of the Related Art
Circuit boards of various types, including semiconductor chip package substrates and circuit cards, utilize conductor lines or traces to convey signals from one point to another. Crosstalk between signal traces due to capacitive and/or inductive coupling can lead to highly undesirable levels of noise propagating through the circuit board. The level of crosstalk is normally dependent upon a variety of factors, such as operating frequency, the dielectric constant of the medium between the crosstalking signal traces as well as the spacing and geometry of the signal traces. With the continuing trend in device manufacture toward increasing miniaturization and higher operating frequencies for signal traces, the problem of crosstalk can place limits on the ability to shrink designs.
Although crosstalk can present problems for various types of signal traces, some types of interfaces may be more prone to interference. Single-ended high speed interfaces such as those used for double data rate and graphics double data rate memory channels may be particularly susceptible to crosstalk due to the ever increasing operating frequencies of these types of channels.
Various conventional techniques exist to address the problem of crosstalk between signal traces. One conventional technique involves the selection of minimum spacing between signal traces above a predetermined level that will yield an acceptable level of crosstalk. Of course this technique places a limit on the ability to increase packing density. Another conventional technique uses guard traces interspersed between signal traces. The guard traces and the signal traces in this conventional technique are located in the same level of the circuit board. The guard traces are electrically connected to a ground plane which is located above or below the level of the signal traces and the interspersed guard traces. Here too, the requirement for a discrete ground plane connected to the guard traces typically limits the positioning of the signal traces on a signal layer and thus presents an impediment to packing density.
The present invention is directed to overcoming or reducing the effects of one or more of the foregoing disadvantages.